Dieback hits Tasmanian forests after dry summer as researchers investigate impact on trees’ future

Dieback hits Tasmanian forests after dry summer as researchers investigate impact on trees’ future
  • PublishedMay 10, 2024

Huge patches of forest in Tasmania have rapidly turned brown over recent months, with many trees dying after a dry summer.

But what this signals for forests in the future as the climate continues to warm is unclear.

From February to the end of April, parts of the state received the lowest rainfall since records began.

Map of Tasmania showing drought conditions in red stretching from the north west to the south east
This map shows the rainfall deficiency for February, March and April.(Supplied: Bureau of Meteorology)

Hobart just endured the third-driest summer on record.

“This is putting amazing stress on our trees,” The Tree Projects lead researcher Jen Sanger told ABC Radio Hobart’s Kylie Baxter.

The extensive sudden tree death in Tasmania is the result of a phenomenon called dieback.

An aerial shot showing a line of orange-brown eucalyptus trees next to greener ones.
The view from above shows the trees impacted by dieback.(Supplied: The Trees Project)

Dieback can be caused by stress due to fungus or other diseases, but in this case it has been driven by drought, Dr Sanger said.

“We’re seeing patches of trees dying, which is really distressing … especially in areas that have got shallower soils or rocky soils or north-facing slopes,” she said.

Surveys have not yet been done to assess the extent of the dieback in Tasmania.

As forests brown on the island state, ecosystems in southern Western Australia and on mainland Australia’s east coast are also experiencing dieback – this includes iconic and culturally significant jarrah and bunya pine forests.

Dieback can make forests vulnerable to bushfire — before the Black Summer bushfires of 2019-20, huge areas of forest in the east of mainland Australia experienced dieback.

And as climate change causes hotter and drier summers, can we expect more tree death in the future?

The future of our forests

The quest to quantify how climate-induced dieback might threaten forests in the future puts University of Tasmania plant physiologist Tim Brodribb in a precarious situation – dangling from a rope 70 metres above the forest floor, attaching scientific instruments to a Eucalyptus regnans. 

Two men wearing hard hats hanging from harnesses up a tall eucalypt tree.
Professor Tim Brodribb (right) is high in the canopy researching the water transport system of the world’s tallest flowering plant.(Supplied: The Trees Project)

Otherwise known as the mountain ash or swamp gum, the species is the tallest flowering plant in the world, sometimes exceeding heights of 100 metres.

When it comes to predicting when dieback could strike, Professor Brodribb says “we really are floating in the unknown”.

“Are half the forests on Earth going to die on their feet or is it 20 per cent?” he said.

“These are really fundamental questions.

“There’s a lot of talk of urgency, but there’s not really any metric on the urgency.”

Man climbing a eucalyptus regnans
Trees have to transport water up from the ground against gravity, so the taller the tree, the greater the challenge.(Supplied: Steven Pearce, The Tree Projects)

Taking the pulse of trees

In the treetops, Professor Brodribb is attaching a device invented by his team — called a ‘cavicam’ — to the dainty leaf of a giant.

This will measure the strength of the tree’s ‘pulse’.

Image of a plastic container in a tree
Seventy metres in the air, this cavicam is ‘taking the pulse’ of a giant eucalypt.(Supplied: University of Tasmania/Tim Brodribb)

Plants create their own food via photosynthesis, where they use energy from the sun and carbon dioxide from the atmosphere to make glucose (a sugar) and oxygen.

During this process water is moved through the vascular system from the roots before evaporating through pores in the leaves.

Man with beard and hard hat climbing a tree with climbing gear
Professor Brodribb hopes his research will help e

The plant’s water transport system, or xylem, doesn’t require the plant to use any energy.

Instead, it relies on the water tension created as water is pulled from the roots to the leaves.

A tree’s pulse is created by the predictable rhythm of increasing and decreasing tension throughout the day.

A resting heartbeat of an adult human is generally between 60 to 100 beats per minute – with no powerful muscular heart, the once-a-day pulse of a tree is far more sedate.

“When the water comes under too much tension, that system breaks and the xylem water snaps,” Professor Brodribb said.

Less water in the ground means less water for the trees, essentially pulling the water in the vascular system tight.

Professor Brodribb likens this to “hypertension” for plants.

“The tree is getting what we call xylem embolisms. It’s like a pulmonary embolism, it’s just a blockage of the vascular system and the tree dies really quickly,” he said.

A scientific slide showing x-ray images of tree trunks.
A scientific slide showing x-ray images of tree trunks.(Supplied: Brendan Choat)

Trees might look like they are moving slowly but zoom in on their leaves and they are as responsive and dynamic as any animal.

Cells in the leaf measure light, humidity and damage, as well as rates of water loss and photosynthesis.

They use this information to decide whether to close their pores to prevent water from escaping or to open them to allow photosynthesis.

“It’s an extraordinary example of coordinated and continuous regulation as these little valves are just constantly maintaining the plant in a safe place,” Professor Brodribb said.

The cavicams, or pulse monitors, give a live view of how the plant is reacting to its environment.

“That’s the pulse of [the tree], and the strength of the pulse tells you how happy the plant is and how much water it’s able to lose,” he said.

Small silver device being attached to a gum leaf
This cavicam being attached by Professor Brodribb will be able to assess the water tension of the leaf, allowing the scientist to better understand what the tree needs to stay healthy.(Supplied: The Tree Projects)

The work Professor Brodribb is doing to understand how much water a tree can lose before it dies is useful for researchers who want to model how climate change may drive dieback, according to Western Sydney University professor Brendan Choat, who also studies dieback.

“Under heat, drought and a combination of heat and drought it helps predict which species are going to die, where and what the implications are for management of forests, and the management national carbon budgets,” Professor Choat said.

Giant trees particularly vulnerable

Plants have to transport water against gravity, so the taller the tree the greater the challenge.

“When you think about the weight of a column of 100 metres of water … that’s a major feat,” Professor Brodribb said.

While attaching a cavicam, Professor Brodribb can get a different perspective on the forest.

“All these giants are emerging all around you, and you notice that all the tops of them are dead,” he said.

the dead top of a tall tree above clouds
It is hard for trees to get water to their top, so the crowns of many of the tallest Eucalyptus regnans have died.(Supplied: The Tree Projects)

Tasmania hosts ancient plants as well as giants, being a refuge for water-loving cool weather species that evolved during the time of the dinosaurs.

“Tasmania is in a particularly fragile position … these are the last places that these fabulously old flora have survived,” Professor Brodribb said.

On the mainland, species can move south or be planted further south in cooler, more favourable conditions, but Tasmania is at “the edge of the world, you can’t go further south”.

Two tall trees against large grey stones
Athrotaxis selaginoides, or King Billy pines, are found only in Tasmania and can grow for thousands of years. They are under threat due to climate change.(Flickr: Athrotaxis selaginoidesNatalie TapsonCC BY-NC-SA 2.0 DEED)


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